In a cylinder block of an internal-combustion engine, a cylinder bore (hereinafter simply referred to as a “bore”), which slides relative to a piston via an oil film, is formed, and a cylinder head is fastened to the cylinder block. FIG. 1 is a plane view which shows a schematic structure of a specific example of a cylinder block 210 used in a four-cylinder engine. FIG. 2 is a side cross sectional view which shows a condition in which a cylinder head 220 is fastened to the cylinder block 210. In FIG. 1, only bores 211 and bolt holes 212 are shown. In the application, a cross section perpendicular to an axial direction is defined as a “cross section”, and a cross section parallel to an axial direction is defined as a “side cross section”.
For example, the cylinder block 210 is made of an Al (aluminum) material, four bores 211 and ten bolt holes 212 are formed at an upper surface of the cylinder block 210. Bolts 230 are fastened to the ten bolt holes 212 of the cylinder block 210 via bolt holes 222 of the cylinder head 220, so that the cylinder head 220 is fixed on the upper surface of the cylinder block 210. A gasket 240 is provided between the cylinder block 210 and the cylinder head 220.
A water jacket 213 is formed between the bore 211 and the bolt hole 212. For example, each bore 211 is formed by a sleeve 214 of a cast iron, a crosshatch is formed by honing on an inner surface of each sleeve 214, and the inner surface thereof is used as a sliding surface. Each bore 211 may be formed by an inner surface of a hole portion formed at the cylinder block 210 instead of providing the sleeve 214.
An inner surface 211A of the bore 211 is subjected to boring and honing, so that as shown in FIG. 3A, the inner surface 211A is formed to have a cylindrical shape of which a side cross section has a straight shape and of which a cross section has an approximately-true circle shape. However, when the cylinder head 220 is fastened by bolts to the upper surface of the cylinder block 210, as shown in FIG. 3B, the inner surface 211A of the bore 211 is deformed so as to be an inner surface 211B. Specifically, an inner diameter of an upper end portion 213 of the inner surface 211A of the bore 211 is larger, and an inner diameter of an intermediate portion 214 of the inner surface 211A of the bore 211 is smaller, so that constriction occurs at the inner surface 211A of the bore 211. Due to this, when a piston slides along the bore 211, friction at the intermediate portion 214 is greater.
In order to improve the cylindricity of the bore 211 after fastening of the cylinder head 220, it has been proposed that the cross section of the bore 211 be machined so as to have a shape which is not a true circle shape in consideration of deformation of the bore 211 which will occur in the fastening of the cylinder head 220 (see Patent Document 1, for example). In the technique of Patent Document 1, the cross section of the bore is postformed so as not to have a true circular shape before the cylinder head is fastened to the cylinder block. In this case, the machined shape (machining shape) of the bore after the postforming is designed such that when the cylinder head is fastened to the cylinder block obtained after the postforming, the bore not having a true circular shape is deformed so as to become similar to an approximately-true circle.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2000-291487.